Chronically instrumented conscious dogs will be studied during the development of congestive heart failure due to mitral regurgitation. New load insensitive measures of systolic performance and cardiac energetics will be used to quantify a large animal, closed-chested model of mitral regurgitation. This chronic model will be used to identify the determinants of myocardial dysfunction and altered energy utilization observed in this disease. These studies should provide insight into the pathophysiology of chronic mitral regurgitation, allowing the development of new therapeutic modalities.